The finite element digital computer code BR-1, developed by the Northrop Corporation, for predicting the effects of internal air blast on combat aircraft structures is modified to include the effects of compressible fluid-structure interaction. The true interaction phenomenon is approximated by the piston theory. The modification enables the code to be used to predict the structural response of aircraft fuel tanks subjected to penetrating bullets and fragments. IT can also be used in many other fluid-structure interaction problems. This report contains the theory, the modifications, and the additional instructions required to operate the modified code, called BR-1HR. The code is operational on the IBM 360/67 in FORTRAN IV, Level H. (Author)